This study investigates the use of granulated cork waste as a partial replacement for sand in cement mortar, aiming to develop thermally efficient, lightweight mortars suitable for non-structural applications in heritage retrofitting. Cork aggregates of three granulometries (≤ 0.5 mm, 0.5–2 mm, and 2–3 mm) were incorporated into mixes replacing sand by weight in dosages ranging from 10% increments up to 80%. A total of 60 prismatic specimens were produced and tested for compressive and flexural strength, thermal conductivity, and capillary water absorption, following relevant EN standards. The results showed that mechanical strength decreased with increasing cork content, but remained within acceptable limits for non-load bearing uses. Medium-sized cork particles (0.5–2 mm) at 40–50% replacement yielded the best compromise between strength and insulation, with thermal conductivity reduced by approximately 20% compared to conventional mortars. Water absorption coefficients remained within a safe range (0.07–0.18 kg/(m2·min⁰⋅5)), indicating that cork does not significantly impair hygric durability. These findings support the potential of cork-cement mortars as viable alternatives for thermal upgrading in heritage contexts where conventional insulation is impractical. Their compatibility with existing lime–cement systems, lightweight nature, and partial use of recycled industrial by-products make them especially suitable for restoration scenarios prioritising reversibility, energy efficiency, and material sustainability.